1. Field of the Invention
The present invention relates to optimal coding of a speech signal, and more particularly, to an apparatus for quantizing spectral envelope and a method therefor with noise robustness for optimally coding the speech signal, under all the environments in which channel errors are not generated and channel errors are generated, and a method therefor.
2. Description of the Related Art
Standardization of speech encoders is proceeding in the US, Japan, and Europe. Most encoders according to the standardization divide speech into a spectral envelope and an excite signal, quantize them, and transfer corresponding bit streams. Therefore, a method of designing a quantizer in which a spectrum envelope is represented by the minimum number of bits is essential. In order to represent the spectral envelope, a linear predictive coding (LPC) is extracted from the speech. In order to efficiently quantize the spectral envelope, the LPC coefficients are converted into line spectrum frequencies (LSFs).
Paliwal and Atal provided a split-vector quantizer (SVQ) in order to quantize the LSFs (refer to "Efficient Vector Quantization of LPC Parameters at 24 bits/frame." IEEE Trans, Speech, audio processing. Vol.1, no.1, pp.3-14, January 1993.). In this method, satisfactory performance is obtained from 24 bits/frame by dividing tenth order LSFs into two or three sub-vectors and separately quantizing the sub-vectors.
Meanwhile, a predictive split-vector quantizer (PSVQ) using an interframe correlation for improving the performance of the SVQ was provided in ITU-T Recommendation G.723.1.
However, this method has a shortcoming in that when a channel error is generated, the error affects the next frame. In order to prevent the error from affecting the next frame, de Marca provided a method of alternately using the SVQ and the PSVQ in odd and even frames. However, this method has lower performance than the PSVQ when no channel error is generated.